The senator steps forward. “We’re not intimidated by you thugs,” he says. The man, saying, “You remind me of my father — I hated my father,” grabs the senator’s head, and thrusts a knife to his face. The senator freezes, eyes wide.

Not your typical Capitol Hill brouhaha. No, this scene is pure Hollywood, straight out of the new Batman movie, “The Dark Knight.” But that really is the senior senator from Vermont: Patrick J. Leahy — Democrat, Judiciary Committee chairman and lifelong Batman fan — has a cameo in the film and gets to be held at knifepoint by Heath Ledger’s Joker.

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Batman became his favorite superhero because “he has no superpowers,” Mr. Leahy said. “He had to use his own brains and his own knowledge. He could have had an entirely different life. As a billionaire, he could have done anything.”

Mr. Leahy had a nonspeaking cameo in the 1997 film “Batman and Robin,” did a voice-over for the part of a governor in a Batman cartoon, and wrote the prefaces for a “Batman” anthology and a Batman comic book about the danger of land mines. Once he was spotted doing wheelies on his grandson’s toy Batmobile down the long marble hall outside his Senate office.

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The filming of Mr. Leahy’s scene in a Chicago restaurant last summer took “all night long,” he said. Mr. Ledger would “punch or throw me halfway across the room,” and Mr. Leahy was propped up by another actor “with an arm like an oak tree” who was “brandishing a gun in my face.”

It took the senator a couple dozen tries before he got his line right.

“We tried it two different ways — one was authoritative, the other one was with a lot of fear in my voice,” Mr. Leahy said. Ultimately, he was directed to act like the prosecutor he once was, with a take-charge attitude.

So how did Mr. Leahy manage to find his character’s motivation? Was he thinking of Vice President Dick Cheney, who in 2004 used profanity to curse Mr. Leahy on the Senate floor?